As the background art of the present technical field, there are PTL 1 and PTL 2. PTL 1 discloses the following technology: “A complex spectrum of observation signals observed with two microphones is obtained, and an inter-microphone phase difference between the complex spectrum of observation signals is calculated for each time frequency. By using the complex spectrum of observation signals obtained from the observation signals observed with one microphone, the inter-microphone phase difference obtained with a feature quantity creation unit, and preliminary information representing the distribution of the complex spectrum of the sound source signals, parameters of a probability model representing the distribution of the complex spectrum of observation signals and the inter-microphone phase difference corresponding to the respective sound sources at each time frequency are estimated. By using a contribution ratio relative to the complex spectrum of observation signals and the inter-microphone phase difference between the respective sound sources at each time frequency obtained from the estimated parameters, the complex spectrum of separation signals is extracted from the contribution ratio and the complex spectrum of observation signals at each time frequency, and then converted into separation signals of a time domain” (refer to Abstract). Moreover, PTL 2 discloses the following technology: “A sound pressure/frequency characteristic measurement device in a voice recognition system incorporates, from a microphone, ambient noise in a period of time with no sound input, and measures a sound pressure of the ambient noise and a time change amount of the frequency characteristic. A voice recognition feasibility determination device determines whether the voice recognition is “favorable”, “possible” or “impossible” based on the measured sound pressure of the ambient noise and the time change amount of the frequency characteristic. The determination result of the voice recognition feasibility determination device is notified to the user by a status notification device.”